Metal-oxide field effect transistors (MOSFETs) generally include a gate electrode, a source region, a drain region, and a body region. The source region and the drain region are of a first conductivity type, and the body region is of a second conductivity type. In some MOSFET devices, the first conductivity type is n-type and the second conductivity type is p-type. In other MOSFET devices, this relationship is reversed. When a MOSFET device is in an on state in response to an applied gate voltage, a channel region forms in the body region below the gate, and between the drain region and the source region. Current flows in the channel region. When the MOSFET device is in an off state, the channel region is not present, and thus current will not flow between the drain region and the source region. However, if a reverse-bias voltage is applied across the drain region and the source region of the MOSFET that exceeds a breakdown voltage, a large uncontrolled current will flow between the source region and the drain region regardless of whether a voltage is applied to the gate electrode. As the reverse-bias voltage increases above the breakdown voltage, an avalanche breakdown event can occur. During the avalanche breakdown event, current through the MOSFET increases at an increasing rate and can quickly exceed a maximum current rating of the MOSFET. As a result, the MOSFET is often damaged or entirely destroyed.
Lateral diffusion MOSFETs (LDMOS) are a class of MOSFETs that additionally include a lateral lightly doped drain (LDD) region to increase breakdown voltage of the semiconductor device as compared to the breakdown voltage of a typical MOSFET. The LDD region is of the same conductivity type as the body region but is doped to a different concentration. Though the LDMOS may have an increased breakdown voltage as compared to a MOSFET, avalanche breakdown events can still occur if a reverse-bias voltage exceeds the breakdown voltage of the LDMOS.